Internal combustion engines having direct fuel injection into the combustion chamber via corresponding (high-pressure) injection valves, and having engine control devices for realizing different operating modes and/or multiple injections or injection sequences per working cycle (e.g. in the form of double or multiple injections), have long been known.
Such combustion methods require a high degree of metering precision on the part of the injection valves, i.e. of the quantity of fuel injected by these valves, in order to enable the exploitation of all advantages of corresponding technologies. This is true above all for multiple injections of very small quantities, which are used above all for starting, for warm running, and for the heating up of catalytic converters. Increasing injection pressures, together with multiple injections, also result in higher demands on the metering precision of injection valves.
The required metering precision, in particular for very small quantities, can be realized only using special methods in which in each case a precise knowledge of the injection behavior of the participating injection valves is required.
From the existing art, it is known to store characteristic quantities of the injection valves in a pre-controlling device, in the form of application parameters. A characteristic, which here reproduces a relation of an injected quantity to the valve controlling duration, is almost linear in the region of longer control durations. This relates above all to the so-called full stroke range in which the injection valve is completely open during the injection process. In the region of smaller control durations, in contrast, there is a significant deviation from linearity. The metering precision decreases significantly in connection with a significant increase in the production spread if the control duration is reduced far enough that the injection valve no longer opens completely, or does so only very briefly.
From German Published Patent Appln. No. 10 2005 051 701, a method is known that makes it possible to describe a deviation of the behavior of an injection valve from the linear relation between control duration and injection duration. For this purpose, test injections are carried out whose durations are first still in the linear range of the mentioned characteristic. The quantity of fuel injected in the context of a regular injection is here reduced by the fuel quantity injected in the context of the measurement injection, in order to avoid influences on the quiet engine running and other disadvantageous effects. The effect of the injections is evaluated by evaluating the air number lambda, i.e. at the engine output side and indirectly. With increasing reduction of the duration of the test injections, a deviation from the linear behavior can be detected via a deviation from the air number that is expected in the case of linearity.
However, there continues to be a need for improved possibilities for assessing the injection behavior of injection valves during engine operation that supply more precise and more reliable results.